fix load-time race condition for high-generic cosmetic filters

potentially causing high-generic cosmetic filters to not be applied
because the MRU cache contains an empty list of high-generic filters
when there is a query from a content script for cosmetic filters
before they are fully loaded and ready.

src/js/cosmetic-filtering.js

@@ -545,7 +545,9 @@ FilterContainer.prototype.freeze = function() {
         }
     }
     this.highlyGeneric.simple.str = µb.arrayFrom(this.highlyGeneric.simple.dict).join(',\n');
+    this.highlyGeneric.simple.mru.reset();
     this.highlyGeneric.complex.str = µb.arrayFrom(this.highlyGeneric.complex.dict).join(',\n');
+    this.highlyGeneric.complex.mru.reset();
 
     this.frozen = true;
 };